1. Field of the Invention
The present invention relates to a diversity receiver and diversity reception method performed in a communication device using a CDMA (code division multiple access) method.
2. Description of the Related Art
In a PDC-based mobile communication system, mobile stations and base stations employing antenna diversity are generally used. The PDC method adopts TDMA (time division multiple access) and so an antenna that is in an optimum reception environment is selected from plural antennas, based on the received field strength in each idle reception interval other than the present slot. Meanwhile, antenna diversity has been proposed also in CDMA techniques. (For example, in Reference 1: JP-A-2001-251230, Reference 2: JP-A-2001-127675, and Reference 3: JP-A-2000-503184.)
However, antenna diversity reception in CDMA is more difficult to achieve than PDC for the following reason. In CDMA, the received signal is code-divided. Therefore, the received field strength of the signal cannot be measured unless the received signal is decoded even to baseband. Therefore, the optimum reception environment cannot be judged. Furthermore, in CDMA, the received signal is discriminated from received signals from other users by code division. The received signal is not time-divided unlike in TDMA. Consequently, the received signal must be kept received and decoded. That is, if one tries to implement a method of TDMA for switching the used antenna during one reception and measuring the strengths at both antennas, the received data will be lost during switching of the antenna.
For the reasons described so far, it is necessary that circuits for encoding the received signal to baseband be equal in number with the antennas to realize antenna diversity in CDMA. Another problem is that the received data will be lost at the timing of antenna switching. These problems must be taken into account. Furthermore, in CDMA, it is only possible to receive signals from a single base station in standby mode, because the signals are code-divided differently for different base stations and the mobile station receives using a code for a sole corresponding base station.
In JP-A-2001-251230 (Reference 1), a receiver and method is proposed which is capable of selecting two antennas providing optimum receive characteristics out of plural antennas for CDMA mobile communication systems. To measure the received field strength in CDMA, it is necessary to decode the received signal even to baseband as mentioned previously. The problem is that a scheme that does not use plural decoding subsystems but compares the strengths by switching the antenna cannot be achieved. Where plural decoding subsystems are provided, there is the problem that the circuit scale is increased.
Furthermore, in JP-A-2001-127675 (Reference 2), a method of making the CDMA base station use antenna diversity is proposed. With this method, all the existing subsystems need to be modified, thus involving exorbitant cost. To avoid this, a method permitting the mobile station alone to cope with the situation is preferably adopted.